Civil Engineering Reference
In-Depth Information
is to operate a pilot filter in parallel with the plant filters. The pilot filter receives raw
water with independent control of treatment chemicals. By monitoring pilot filter ef-
fluent turbidities and making appropriate adjustments in chemical treatment, optimum
filterability of the water can be obtained. The use of raw water in the pilot filter
provides the necessary lead time to anticipate plant requirements, particularly on raw-
water supplies that experience rapid changes in water quality. Maximum filterability
is much more important to production of a water of maximum clarity (minimum tur-
bidity) than is maximum turbidity reduction prior to filtration.
Some properties of the filter bed that affect filtration efficiency are: the size and
shape of the grains, the porosity of the bed (or the hydraulic radius of the pore space),
the arrangement of grains (whether from fine-to-coarse or coarse-to-fine), the depth of
the bed, and the headloss through the bed. In general, filter efficiency increases with
smaller grain size, lower porosity, and greater bed depth. Coarse-to-fine filters contain
much more storage space for materials removed from the water than do fine-to-coarse
filters, and permit the practical use of much finer materials in the bottom of the bed
than can be tolerated at the top of a fine-to-coarse filter.
The total surface area of filter media grains is important because it represents the
total area available for adsorption of floc. Figure 12-4 summarizes the total surface
area of different sizes and depths of filter media.
14
Because of the greater total surface
area of the grains, smaller grain size, and lower porosity, the coarse-to-fine filter is
more efficient than the fine-to-coarse filter. The much greater total grain surface area
and the smaller grain size provided by mixed media as compared to dual media account
for the greater resistance to breakthrough provided by mixed media. A mixed-media
6000
5000
4000
3000
2000
1000
0
10" Coal
(1.30 mm)
10" Coal
(0.77 mm)
10" Sand
(0.45 mm)
10" Garnet
(0.30 mm)
20" Coal
(1.0 mm)
10" Sand
(0.30 mm)
18" Coal
(1.0 mm)
9" Sand
(0.45 mm)
3" Garnet
(0.30 mm)
Fig. 12-4.
Filter grain surface of different filter media sizes and depths (
Source:
Reference 14)
